CONSTRUCTION OF THE DSS SYNTHESIZER

The following notes and pictures describe the main aspects of the synthesizer construction. In the absence of a PCB layout the information given here should be useful to anyone considering building this system, alternatively parts of this project may find application in other designs.

THE SHAFT ENCODER

In a digital tuning system the shaft encoder is often one of the most difficult and expensive components to find. The low cost simple 16 or 32 step contact type encoders commonly used in audio volume controls would not be suitable for this design. To develop a "smooth tuning" tuning feel an encoder with several hundred steps was required.  Ready assembled units can be found but these are expensive so it was decided to have a go at building one from basic parts. Fortunately a reasonably priced 512 step code wheel and optical assembly is manufactured by HP-Agilent. The HEDS9100 is available from Farnell at around £20 UK. This was assembled into a chassis made from copper laminate board into which a 1/4" shaft made from brass tube (any model shop) was mounted through two 1/4" instrumentation bearings from RS. The brass tube has to be reduced slightly by running down with emery paper to allow the tube to be inserted into the bearings with an interference fit. The shaft assembly was completed by placing the encoder wheel onto the shaft along with the bearings which were carefully secured with epoxy in the chassis frame. The optical reader was mounted on a small scarp of PCB which was soldered in place on the chassis. One feature of the HEDS9100 is that it is tolerant of misaligned, hence precise positioning is not necessary. Notre that in the picture the highly reflective metal code wheel appears transparent, it's not this is a reflection. The mounting boss with it's allen head set screw is out of sight on the other side of the code wheel. Three screws (visible on the facing end plate) are used to secure the assembly to the transceiver front panel. The bearing assembly runs so freely that the shaft will "creep" in certain knob positions. This is caused by gravity acting on the asymmetric weight distribution of the front panel knob's construction. An length of shaft has been left to the rear of the encoder to allow an identical knob to be fitted inside the transceiver to counterbalance the offset weight. This was not fitted when these pictures were taken.

THE SYNTHESIZER MODULE

Arguably the most intricate part of the whole system - perhaps I've over engineered this section, many designs I've seen are not as elaborate and seem to work fine, but I wanted to ensure adequate screening and good mechanical strength.

 The DDS assembly is built onto a piece of copper laminate which sits inside a diecast box (probably not necessary). The silver unit on left is a packaged 125MHz HCMOS oscillator module. The DDS chip is in direct centre of the a 29 pin header into which a thin piece of copper. This mounting technique was described by Peter Rhodes in his excellent DDS project featured in the RadCom January to May 99 editions. Bottom right is the output filter, above this are two MMIC amplifiers. The RF output socket is top right just to the right of one of the main board mounting screws.

THE PROCESSORS AND DISPLAY

If I was building this synthesizer again this is one area where I would definitely make changes. I used a pair single sided "utility" PCB's I developed for the 16F84 for use in a range of projects. Unfortunately there are errors with the track layouts of these and so I haven't provided the artwork. Whilst this approach seemed convenient it does give rise to a considerable amount of wiring. Surprisingly (and to my relief) this doesn't seem to cause interference with the transceiver IF system. The change I would make here it to mount both processors on a board that directly mounts onto the back of the LCD display module. This would greatly tidy up construction and keep peripheral wiring to a minimum. The first picture shows the IF Module in mounted in the transceiver unit with the display to the right and the shaft encoder just visible to the rear. In the second the dual PIC processor boards, shaft encoder, and synthesizer module can be seen. The final picture is a general view of the transceiver unit's front panel. Controls from left to right are On/Off AF Gain, USB/LSB (By shifting DDS output |IF+LO|,| IF-LO|), AGC Time Constant (Yet to be wired in) and the main DDS function control (Calibrate, 1Hz,10Hz, Band Change - via main tuning control)